1. Field of the Invention
This invention deals with an improvement to vacuum field effect transistors (FETs) having field emission cathode structures, and with circuits employing these devices.
2. Description of the Related Art
The first vacuum FET (also called a microelectronic tube) was announced in 1986 by Dr. Henry Gray of the Naval Research Laboratory. The announcement generated a considerable amount of interest, and stimulated further developments in the technology on an international scale.
The vacuum FET is basically an electron tube fabricated on a microelectronic scale using semiconductor processing techniques. Due to extremely small dimensions of the device (cathode tip to anode spacings are typically 0.5 micrometer), the mean free path of the electrons can be greater than the cathode tip to anode spacing, allowing vacuum tube type operation at pressures up to 1/100 to 1 atmosphere. The vacuum FET has distinct advantages over other semiconductor devices in terms of radiation hardness, extreme temperature insensitivity, very high speed operation and power efficiency, small and light weight packages, and relatively low cost. The development of the technology is reviewed in an article by Kathy Skidmore, "The Comeback of the Vacuum Tube: Will Semiconductor Versions Supplement Transistors?", Semiconductor International, August 1988, pages 15-18. The basic structure is described in U.S. Pat. No. 4,721,885 to Brodie.
The structure of the current vacuum FET is shown in FIG. 1. The device is formed on a substrate 2, which may be ceramic, glass, metal or silicon. A conductive layer 4 is deposited over the substrate. The electron emitter 6 may be formed either from silicon, or from a refractory metal such as molybdenum or tungsten. An insulating dielectric layer 8 such as silicon dioxide is then deposited over the conductive layer 4, followed by the deposition of a metal film 10 which functions as the gate electrode. Silicon is preferred for the conductive layer 4 because of the ease of growing the oxide 8 with a high dielectric strength over it. Dielectric layer 8 and gate electrode 10 are etched to form a gap around the periphery of electron emitter 6, with the tip of the electron emitter centered in the opening and approximately level with the upper gate electrode surface to maximize the electric field at the tip.
A second insulating layer 12 is then deposited over gate electrode 10 and etched to expose the electron emitter. A metal film 14 is deposited over dielectric layer 12 to form the anode or collector. The collector 14 covers an interior chamber 16 within the device, which is maintained at a partial vacuum generally within the range of 0.01 to 1 atmosphere.
In operation, a collector voltage V.sub.C is applied to collector plate 14 to extract an electron stream from emitter 6. The gate electrode 10 is biased by a gate control voltage V.sub.G to modulate the emitted current in an amplifier mode, or to switch the current emission on and off completely in a logic switching mode.
While vacuum FET technology has the distinct advantages discussed above, the standard device of FIG. 1 merely provides a basic amplifying or switching element. As with standard semiconductor transistors, a number of such devices must be connected together to perform higher level functions such as multiplexing, frequency multiplication, logical AND or EXCLUSIVE OR. In this sense, the basic vacuum FET provides no new functionality over existing solid state technologies.